This type of method for measuring a glass panel's sagging is prior known from Finnish patent application FI-912871. That comprises monitoring the sagging with a measuring instrument for applying a laser beam to the surface of a glass panel and a CCD camera is used for tracking the distance of a confluence point between glass and laser beam from a reference plane. This arrangement requires a lot of space and sets constraints regarding the choice of a measuring point. An angle of reflection required for the laser beam calls for two holes in the roof of an oven for each measuring point and accuracy is not sufficient because of the angle of reflection.
Patent application WO 02/23124 discloses a method, in which a line camera is used for tracking the angle of reflection of a light beam reflected from glass. The angle of reflection is highly sensitive to a possible surface defect in glass and to a local curvature defect in glass, whereby the angle of reflection provides a defective measuring result. This method may also involve measuring the distance of a reflection point from the camera by applying a second light source and the triangulation principle.
In addition to the above, there are a host of prior known methods, disclosed for example in publications FI 850120 and FI 98757, wherein light beams reflected from an arched or arching glass panel are monitored in various ways by means of a line or matrix camera or cameras. These methods are hampered by problems similar to those found in the method disclosed in publication WO 02/23124.
It is an object of the present invention to eliminate or substantially alleviate the above drawbacks.